This invention relates generally to a tandem drive axle set and, more particularly, to a forward drive axle from a tandem drive axle set that does not include an inter-axle differential.
A tandem drive axle set is used to distribute rotational power from a driveline input to a set of forward and rear wheels through a forward drive axle assembly and a rear drive axle assembly, respectively. Traditionally, the tandem drive axle set includes an inter-axle differential (IAD) assembly that divides the driving input force between the forward and rear drive axles. A set of helical gears is used to transfer a portion of the driving input force from the IAD to a ring and pinion gear set for the forward drive axle. The IAD transmits the remaining portion of the driving input force to a through shaft, which sends the driving force back to the rear drive axle. The through shaft is coupled to an output from the forward drive axle.
A connecting driveshaft is coupled to the output from the forward drive axle at one end and an input to the rear drive axle at an opposite end. The rear drive axle assembly includes a ring and pinion gear set that drives the rear wheels via input from the connecting shaft. Because the helical gears are necessary in the forward drive axle, the axis of the driveline input to the forward drive axle is offset from the pinion gear axis in the forward drive axle by the centerline-to-centerline distance of the helical gears. Therefore, the output of the forward drive axle assembly is on the same axis as the driveline input while the input of the rear drive axle assembly is on the same axis as the forward drive axle pinion gear.
This difference in axis height between the forward axle output to the rear axle input requires different axle pinion angles to be utilized in order to set the driveline angles in the u-joints used in the driveline. Setting and maintaining the driveline angles is difficult. When the driveline angles at the u-joints are not the same it creates adverse torsional loading and vibrations in the drivetrain assembly. Such torsional loading and vibrations can lead to premature failure of the drivetrain assembly. Even when the driveline angles are properly set at the factory, the air-ride suspensions commonly found in heavy duty trucks can alter the driveline working angles in an adverse manner.
One solution is to utilize a hollow pinion gear in the forward drive axle assembly as described in U.S. Pat. No. 6,200,240 assigned to the assignee of the present invention. In this configuration, the through shaft extends from the inter-axle differential and through the hollow pinion gear. The hollow pinion gear drives the main differential in the forward drive axle assembly. The through shaft extends toward the rear drive axle assembly which utilizes a traditional pinion gear to drive a rear differential. This configuration allows a common axis to be shared by the input to the forward drive axle assembly and the input to the rear drive axle assembly.
In certain vehicle applications, it is advantageous to utilize a common axis for the inputs to the forward and rear drive axles without requiring an IAD assembly. Requiring an IAD assembly to achieve a common input axis benefit unnecessarily increases cost and results in unwanted additional weight to the vehicle. On the other hand, certain applications cannot be configured to have a common axis for the forward and rear pinions. However, it is still advantageous to reduce the vertical height difference between forward and rear pinions as much as possible.
Thus, it is desirable to provide a tandem axle set configuration that does not require an IAD assembly but which still provides a common axis to be shared by the input to the forward drive axle assembly and the input to the rear drive axle assembly or which provides a significantly reduced vertical height between the pinion inputs. The elimination of the IAD from tandem drive axle set should be accomplished without requiring significant modification of existing components as well as overcoming the other above mentioned deficiencies with the prior art.